1. Field of the Invention
This invention relates to a focusing screen interchanging device in single lens reflex cameras, and more particularly to the construction of a focusing screen and a fixture member therefor to be fitted in camera body in the form of a unit so that the attachment of the focusing unit to the camera body is made easy and safe to perform without contamination.
2. Description of the Prior Art
When it is desired to interchange the focusing screen in a single lens reflex camera, two methods have been put into practice in the past. One involves a removable finder housing containing a pentaprism which is attached to the camera body to make it possible to interchange the focusing screen through an opening provided in the top panel of the camera housing. The other involves application to a camera with a fixed finder housing wherein the interchanging of the focusing screen is performed through an opening in the amount on which the interchangeable objective lens is later to be attached.
The former of the above-described interchanging methods has found use in the conventional type of focusing screen interchanging device an example of which is shown in FIG. 1 constructed as a unitary member with a focusing screen 3 and a fixture member 2. When it is desired to attach this to the camera body, the unit it placed into a receptor 1a within the interior of the camera housing 1. To detach the focusing screen 3, the operator must either pull out the focusing screen unit (comprising the above-described focusing screen 3 and the focusing screen fixture member 2) from the receptor 1a or turn the camera body 1 upside down, thus causing the focusing screen unit to fall out of the receptor 1a. Such a form of the device, however, requires not only difficult handling of the focusing screen during attaching and detaching operation, or when interchanging the unit, but also requires the operator to directly touch the focusing screen unit with his fingers, thereby producing a disadvantage in that either or both of the Fresnel and matted surfaces of the focusing screen is or are very often contaminated by finger prints, or scratched with the result that the finger optical system and light metering system malfunction.
The latter method of handling the focusing screen through the opening in the lens mount also applies to a conventional type of device, an example of which is shown in FIG. 2, where the focusing screen 3 only is made interchangeable as is common practice in the prior art, while the fixture member 2 for the focusing screen 3 is pivotally mounted on the framework of the camera body 1. When the focusing screen is to be attached, this member 2 is turned to a position shown by dashed lines, then the focusing screen 3 is put on the fixture member 2 by the use of tweezers or the like, and then the fixture member 2 with the focusing screen 3 thereon is returned to the operative position shown by the solid lines by a finger of the operator. Since, however, the focusing screen 3 is generally made of a plastic plate very susceptible to scratches, such method gives rise to a very difficult problem that the fixture member 2 is imparted with a rigidity strong enough to assure the accurate fitting of the focusing screen 3 under the bottom face of the pentaprism 14 within the camera housing 1 at a certain position without causing the mechanical complexity to be increased. Further, the use of the finger in moving the focusing screen fixture member 3 to the prescribed position and clamping it therein leads to a high possibility of occurrence of a finger print or scratches on or in the focusing screen 3 and the mirror 11 which also produce bad influence on the light metering system and the view-field of the finder. Another disadvantage is that the focusing screen 3 is liable to be broken by mishandling when interchanged so that the required precision of fixture of the focusing screen cannot often be reproduced. Still another disadvantage is that when the necessity of repairing or exchanging the focusing screen fixture member 2 with a new one arises, it entails great cost, and reestablishment of the original precision after the reassembly operation is very difficult to achieve.